Why isn't Rigify adding constraints to the bones it generates?
The grey bones you see is a predefined armature template riggify uses to generate the actual armature. 

The template armature ( grey bones ) are there for you to line them up with your mesh so that when you generate the rig everything is in the right position and you can start weight painting or autoweighting directly.

If you want to see the complete setup you can enable the layers in the armature properties. The bones you start out with can be discarded, these are no longer used beyond the point of generating the actual armature.